In nuclear power stations, and in particular in the reactor buildings of nuclear boilers, various different types of electrical equipment are installed such as actuators, sensors, etc., which are used for monitoring the various operating parameters of the reactor and for controlling the reactor. These various sensors and actuators are designed and built to operate correctly under all operating conditions, normal or accidental, so as to enable the important parameters of the reactor and its auxiliary circuits within the confinement building to be monitored and controlled continuously.
The accidental operating conditions taken into consideration are conditions resulting from accidents selected because of their severity, such as earthquakes, and more particularly accidents which give rise to a break in the primary cooling circuit of the boiler.
The various items of electrical equipment must continue to operate during and after such conditions, i.e. they must withstand the very severe ambient conditions that then arise. They are therefore designed to withstand accidental conditions during the theoretical lifetime of a nuclear boiler, and they do indeed withstand them. However, there remains a problem at the various connection points between said equipment and the cables connecting them to the various devices which issue commands and which receive measurement signals.
These cables use divided mineral insulation, for example powered megnesia or alumina, together with stainless steel sheathing which enables them to perform their transmission function reliably even in the presence of radiation, high temperatures, and certain kinds of deformation or traction.
Such mineral insulation loses its insulating ability if it is exposed to moisture. Along the length of the cable, the mineral insulation is effectively protected from moisture by the sheath. However, this sheath is necessarily interrupted at each end of the cable in order to allow electric connections to be made to the cable. That is why, in order to avoid the risk of losing electrical insulation by ingress of moisture into the insulating layer of the cable from its ends, it is conventional to protect each exposed end. This is done by means of a protective alumina end-fitting which covers the exposed end of the insulating layer.
Further, the terminal block, i.e. the set of connection terminals to which various cables are to be connected, is disposed in a connection box which, conventionally, is made in sealed manner in order to prevent moisture from coming into contact with the terminals and thus coming into contact with the protective end fitting.
The end of the insulating layer is thus protected by two barriers: a close barrier, namely the protective end-fitting; and a more distant barrier, namely the connection box.
However, it appears, that this double protection is not as safe as could be desired.
More precisely, it has appeared that the technology usable in practice for making boxes for electrical terminals simply and cheaply does not guarantee that they will remain sealed after certain types of accident. This can be understood by considering the values assumed for certain parameters inside the reactor building after a break in the primary pipework, where such a break is considered as being the most damaging form of accident;
temperature: about 156.degree. C. PA1 pressure : 5.5 bars PA1 humidity : greater than 80% PA1 chemical spray: pH 9.5.
Under such accident and post-accident conditions, the sealing of connection boxes is not maintained. As a result water vapor penetrates therein under the effect of the pressure difference between the inside and the outside. This water vapor condenses inside the boxes without being able to escape, and moisture penetrates into the ends of the cables. The resulting degradation in the electrical characteristics of the cables in the vicinity of the connections then gives rise to considerable disturbances in the transmitted signals. It becomes difficult, if not totally impossible, to control the reactor after such degradation.
Comparable problems may occur in other industries, in particular in chemical works or at sea.
The aim of the present invention is to enable a mineral insulated electric cable to be connected in a simple, cheap, and safe way while retaining the insulating ability of the cable even under severe conditions which may include spraying with highly conductive water, a confined atmosphere, and large pressure differences.